The present invention relates generally to electronic devices for protection against short circuits in circuits containing semiconductor power elements. In particular, the semiconductor circuits to be protected include unitary voltage gain signal transducer circuits such as the final stages of audio power amplifiers or voltage regulators.
A state-of-the-art protection device is described, for example, in U.S. Pat. No. 3,792,316, assigned to the assignee of the instant U.S. patent Application. This protection device, normally used to protect the final power element of a monolithically integrated amplifier circuit, includes circuit means to read and analyze the value of the current flowing through the power element itself and the value of the voltage at selected terminals of the power element. The information thus obtained on the instantaneous power dissipated by the power element is used to activate, when certain power values are reached, a threshold circuit which reduces the current in the element itself to a maximum value which is a function of the voltage value present at the selected terminals. As soon as this power, due to an accidental overload such as a short circuit at the output terminal, reaches dangerous levels for the power element, the protection circuit is activated keeping the dissipated power within safety limits defined by a characteristic curve of the transistor itself.
The protection devices of the kind described above perform very well the task of preventing the destruction of an integrated circuit power element due to accidental overload. However, they do not prevent the components from working at maximum permissible power during the entire time of overload conditions.
However, since the users of integrated circuits with power elements generally, for economic reasons, select the external power sinks for discharging the heat created by such elements according to normal operating condition requirements, under conditions of prolonged short circuit, a danger continues, in spite of the protection, both of damaging the integrated circuit and of overheating the whole apparatus of which the circuit is a part, with consequent heavy damage to the structures adjacent to the apparatus itself.
On the other hand, it is not advantageous to decrease the maximum level of current in the power element by lowering the protection intervention threshold, because the dynamic performance of the circuit would be unnecessarily limited under normal operating conditions.
A typical example where the above-mentioned protection devices prove to be insufficient relates to audio amplifiers which are mounted in motor vehicles, there being a large short circuit risk because the "ground" is the vehicle itself. Therefore, as soon as the electrical connecting cables touch a metal part of the car because of carelessness during installation or defects in insulation, they can cause a short circuit and therefore a high flow of current through the power element, the load impedance, or both. An accidental short circuit, if not immediately eliminated, will not only cause the car battery to quickly discharge, but can also cause a fire.
A second type of a state-of-the-art protection device, described in the U.S. Pat. No. 4,368,436 assigned to the assignee of the instant U.S. Patent Application protects against overloading unitary voltage gain signal transducer circuits, such as the final stages of audio amplifiers. This protection device limits its operation to short circuit conditions and requires a negligible power dissipation for the protected device.
This protection device includes a threshold comparator, whose first and second terminals are coupled respectively to the signal input terminal and to the transducer circuit output terminal, and whose output terminal is coupled to a transducer circuit inhibit terminal. The threshold of the comparator is adjusted in such a way that the comparator generates, at the output terminal, a signal inhibiting the transducer circuit when the difference between input and output voltage of such circuit exceeds a predetermined value, as is the case with a short circuit at the output terminal.
The threshold voltage value is chosen in relation to the maximum dissipation physically allowable for the power components and the characteristics of normal load.
The restoration of the comparator initial conditions after the overload conditions have been eliminated, in order for the transducer circuit to return to normal operation, can be automatic or can be controlled manually. The restoration is obtained by increasing the transducer output voltage until the difference between input and output voltages is reduced below the predetermined threshold value. In the above-mentioned application, there is the description of an implementation example where this function is performed automatically by a current generator inserted between the output terminal coupled to the load and the higher potential supply terminal. In fact, the current thus delivered to the load impedance makes it possible to obtain a sufficient voltage at the output terminal so that the comparator may cease to inhibit the transducer circuit's normal operation when the short circuit has been eliminated.
However, this method of obtaining an automatic switching back on of the circuit is counterproductive when the load impedance is AC coupled to the output terminal by means of a condenser. The threshold comparator can no longer be sensitive to a short circuit of the load downstream from the condenser, because the DC isolated transducer circuit output terminal, is kept at too high a potential by the current supplied by the current generator. If the current generator is omitted, the final stage's stray current can be theoretically sufficient, after removal of the short circuit, to bring the output terminal to the voltage enabling the transducer circuit to be restored to normal operation.
In practice, however, this restoration is not certain, or it happens with a delay that cannot be determined. A protection device of the type indicated above is also inefficient, independent of the presence or lack of circuit means for the automatic switching back on of the protected circuit, when a double supply is used and a load is DC connected between the output terminal and the ground potential, the ground potentials being intermediate between the potential of the two supply terminals furthermore a load, inserted between the output terminals of two transducer circuits that are in phase opposition in a bridge circuit arrangement and in which each transducer has a protection device of the kind just described, is not protected at all in the case of a short circuit at its end terminals.